Production of ferrochromium



Patented Apr. 15, 1941 UNITED STATES PATENT OFFICE' PRODUCTION OF FEBROCHROMUM Percy H. Royster, Bethesda, Md. Application January 23, 1939, Serial No. 252348 4 Claims.

This invention relates to the pyrometallurgy of ores containing chromium, and is concerned with the production of alloys containing chromium-e. g., an alloy of chromium, iron and carbon of relatively high (i. e., 30% or more) chromium content-by smelting, in the blast furnace, an ore (or metallurgical equivalent thereof) containing chromium and another metal.

Heretofore ferro-chromium and other alloys containing chromium have been produced exclusively in the electric furnace. It had been thought that the conversion of chromium oxide to chromium carbide is chemically very dificult. Moreover, it had been thought by metallurgists that the conversion of chrome ores into chromium alloys could not, with any practical suecess, be carried out in the blast furnace because of a supposed necessity for employment of smelting temperatures of a higher order than were attainable in a blast furnace in order that conditions of thermodynamic equilibrium would be favorable to the reduction of the oxide, 1. e., favorable for causing the reaction expressed by the equation Cr203+3C -2Cr+3CO 1) to progress from left to right.

The reason for the above-recited openions is not clear. The reaction of the above equation takes place at about 2137 F., or, at one atmosphere pressure, at a temperature of only about 1969" F. The standard enthalpy change, AH, of Equation 1 is 155,760 calories per mol, and the standard change of entropy, AS, is 113.90 entropy units (a u.) per mol. From the publication by Kelley (Bur. Mines Bulletin #394, 1936) it is possible to calculate thermodynamically that at one atmosphere, equilibrium in Equation 1 is established at 2032 F. (the temperature at one atmosphere being the ratio of enthalpy change to entropy change), the enthalpy change, AH, being 177,850 calories per mol and the entropy change, AS", being 126.25 e. u. per mol. As reduction takes place according to Equation 1, carburization of the metallic chromium takes place according to the equation 5Cr+2C=Cr5Oz (2) and the actual mechanism of the reaction between CrzO: and carbon is a reduction of the oxide and a carburization of the metallic chromium. The efiect of this carburization-Equation 2-is to decrease AH of Equation 1 by 1.5% and to increase AS by 3.4%. For simplicity,

Equation 2 is omitted from the following exposition, and the terms metallization, "reduction" and deoxidization of CraOa are used to mean the conversion of chromium oxide into its thermodynamic equilibrium reduction product, which latter is hereinafter indiscriminately called metal, metallic product, carbide," and/or alloy.

In the accompanying drawings Figs. 1 to 5 inclusive are phase diagrams of binary systems hereinafter referred to, and Fig. 6 is-ea. diagram illustrating the relationship between maximum temperature of the furnace gases at the combustion zone of a blast furnace and blast temperature.

In ores containing iron and suficient chr0- mium to produce an alloy of iron and chromium of high enough chromium content to warrant the designation fer'ro-chromium, the mineral most frequently encountered is chromite or ferrous chromite, FeC1204 or FeO.Cr20a. Pure chromite melts at 3800 F. The ratio of FeO to CrzOa varies widely in chromite ores from difierent sources. The binary system Fe0.Cr2Oa shown in Figure l of the accompanying drawings exhibits no low temperature points on the liquidus, the eutectic between FeCrzOs and CIzOs, the lowest melting temperature on the right hand half of Figure 1 being 3640 F. The eutectic between FeO and FeCmOi is not of any practical importance, since its composition is about 98% FeO and only about 2% CH0: (1. e., a composition not encountered in the production of ferro-chromium) When ores containing chromite, of whatever ratio of FeO to CIzOa, are charged into a blast furnace they encounter, during their descent through the furnace shaft, ascending furnace gases which are rich in CO and poor in CO2. During such descent the greater part of whatever FeO may initially have been present in the ore is converted to metal (Fe, or FeaC, or both), which event leaves the oxide phase substantially free from FeO although the metallic iron may still be retained mechanically in position in the original lump of ore. As reduction of the FeO content progresses, the CIzOa content of the ore particle or lump moves from left to right, in the diagram shown in Figure 1 approaching the composition of iron-free CrzO: with its high melting point of 4,130 F.

Usual eflorts to flux these refractory ores fail to produce a low melting mineral particle. This may be illustrated in the binary system 0110-- CraOa by the phase diagram shown in Figure 2.

From this latter it will be seen that calcium chromite (CaCrzO; or CaO.Cr:Oa) long called infusible," has a melting point of 4,000 F. and that the binary system exhibits a minimum liquidus temperature of 3430 F. (51% CaO and 49% CraOa, by weight). In practice the amount of CaO which would be added to the CraOa would be less than pound 'per pound, so that the "effective minimum liquidus temperature in the system is about 3830 F. (for the eutectic between lime and calcium chromite at 21 parts of CaO to 79 parts of CrzOa). Further increase of CaO, such as the suggested scheme for replacement of FeO by CaO would call for-i. e., one mol of CaO to each .one mol of CraOa to" form calcium chromite-raises the liquidus temperature to 4,000 F.- None of the temperatures in Figures 1 and 2 are within the range of the blast furnace as the same. is now known.

Commercial chromite ores are not simply com:

.MgOL'mOa shows no compound or eutectic but only complete solid solution, with a minimum liquidus temperature of 4130 F. (CraOa'itself) The same phenomenon (i. e., absence of compounds and eutectics, and a complete series of solid solutions) is found in the binary system A1203.C12O3, the minimum liquidus temperature of this latter system being 3722 F, (i. e., the melting point of A1203). SiOa, the third of the imp ortant gangue oxides, does not even form a system with CraOa. Mixtures of S102 and Crzs have been found to show no solubility of either oxide in the other'at as high as 3852 F. and above; When both oxides are molten together they appear in the melt as immiscible liquids.

The CrzOa content of chromite ores is readily reducible, but the reaction between carbon and CH0: can take place only when chemically reactive contact is established-i. e., only when the carbon atom and the CH0: molecule are brought within a distance of centimeters, which means that the CraOa material must be suiliciently liquefied to flow across the surface of the carbon particle.

The above facts probably account for the general opinion that chrome ores could not be smelted in the blast furnace to produce ferro-chromium. They showthat the gangue oxides present in chrome ore do not flux Ci'aOa; also, that FeO and CH0: do not produce a compound of low enough melting point to come within the blast furnace range. In the blast furnace shaft the FeO is reduced early in the ores descent.

The average temperature of coke lumps arriving at the combustion zone of the blast furnace is only 3085" F., ranging between 2993- and arrival at the blast entrance and the solid line indicates the temperature of the gaseous combustion products leaving the combustion zone (i. e., the highest temperature attained in the blast furnace), The blast temperatures given in the references above range between 1030 F. and 1250 F., with about 1450 F. as a maximum. The highest blast temperature known to have been employed in blast furnace practice is about 1600 F. to possibly 1700 F., but use of such a high temperature blast has not been customary or regular. gilt ttlielhmgrefriquently employed 1450 F. hot

as e uy re emperature is, c ure 6, about 3340 F. ac ordmg to Fig 'I'he problem of smelting chrome ores producing ferro-chromium therefrom, in th e l a i furnace is seen, from the above, to be diflicult not because of any property of irreducibility of the CH0: but purely because of the mechanical difiiculty of liquefying the chromium-bearing material in order that the CrzOa molecule may be brought into chemically reactive. contact with solid carbon.

I have found that for practical operation of the blast furnace as a means for smelting many chrome ores to ferro-chromium blast temperatures maerially in excess of those customarily used in blast furnace practice should be employed The process of the present invention is one of smelting a chrome ore, having a substantially high content of chromium to yield a chromium V alloy containing more than 35% chromium, in a the ore particles into he combustion zone.

While it is impossible to assign any fixed cormbustion zone temperature to 'be maintained in the blast furnace smelting of chrome ores-since varying amounts of gangue oxides change this temperature over many hundred degrees-the liquefying temperature of any particular ore can be ascertained (as by experiment): with the liquefying temperature ascertained one can, by consultation of the diagram in Figure 6, readily determine the blast temperature necessary for a 3189 F. (Bur. Mines Technical papers 391 and 393; Iron Trade Review,-No. 1919, p. 1452; Blast Furnace 8: Steel Plant, vol, 12, p. 154), and the average hearth temperature (1. e., average of the temperatures of metal and of slag as measured by an optical pyrometer corrected for emissivity) is only 2725 F. These observed temperatures agree very well with the calculated temperatures shown in Figure 6, wherein the lower dotted line indicates the temperature of coke lumps at their furnace charge including that particular chrome ore.

The invention will now be described with greater particularity in the following illustrations:

Illustration 1 A chromite, or chrome. ore from New Caledonia has the analysis: Cram-54.5%; -FeOl7.7%; MgO-8.0%; AhO::11.1% Bios-3.1%; with met-allies; charged into the blast furnace, the CO in the furnace gases reduces the 'FeO content before the ore particles reach the combustion zone,'thus eliminating iron from the oxide phase, and producing a .mineral of composition: Cra057l.l%; MgO-10.3%; Al:Oa-14.5%; and S1024.1%. Considering at the start 810: as a minor diluent and omitting it from instant consideration, the oxide combination lies in the ternary system MgO--Crz0aAlaOa, and its composition projected from the ternary system data the MgO-hhO; binary boundary exhibits an Mao-A; com- Cr37.3%, and Flt-13.8%. When.

position of 42 parts of M30 to 58 parts of A1203. The phase diagram for MgOAl2O: is shown in Figure 3. The llquidus temperature is 3805 F. and the solldus is 3680 F., the melt at 3805 having about one-third of its weight as primary crys tale of spine] (MgAlzOO suspended in the spinelmagnesia melt. Drawing a line from the 42MgO-58Al203 point on the MgO-AlzOa binary across the ternary system to the CrsOa apex gives a pseudo-binary system consisting of CizOa as one component and the MgO-AiaOa combination as the second. The llquidus of this pseudo-binary is shown in Figure 4, with a minimum temperature (marked "eutectic) at 35-10" F. occurring at 34% ClzOa (by weight. This is not a true ternary eutectic (l. e., quintuple point), but a phase boundary of spine] in the ternary system. The ternary composition is: Mg10.7%; Cr203--74.2%, A120a15.1%; and its llquidus temperature is 3950 F., and its solldus 3510 (pseudo-binary eutectic). At 3510" F. the ternary system consists of about 60 per cent primary crystals of CizOa suspended in the ternary melt. The effect of the 8.54 molar per cent of S: (ignored until now) is to lower the solidus temperature by 138 F., since the entropy of fusion of the ternary melt is 10.92 e. u. per moi. The minimum point of liquefaction of the ore particle is thus 3372 F. Reference to Figure 6 shows that a blast temperature of 1500 F. is Just sumcient to attain a solid particle temperature in the combustion zone of 3372 F. and it is true that some trifling liquefaction of the New Caledonian ore will take place in a blast furnace blown with air preheated to 1500 F. In actual engineering practice, where rates of descent of the charge column of three-eigh-ths of an inch per minute to one inch per minute are employed, the total length of time at which the oreparticle remains at the elevated temperature is limited. The minimum temperature given above represents a sex-tuple point in the quaternary system MgO-Cr;Oa-AlzOs-Si0s, and the viscosity of the liquid itself is very high. Although it is true, scientifically, that liquefaction begins at 3373, I have found in actual furnace practice that a superheat of 100 to 200 F. above the solidus is desirable for satisfactory furnace operation in or der that the liquefied ore particles have a free flowing property. At 100 F. superheat, via,

3473 F. combustion zone temperature, a. blast temperature of 1650 F. is required according to Figure 6 and at 200 superheat, viz., 3573 F. combustion zone temperature, a blast temperature of 1800 F. is required. I prefer to operate with the higher superheat whenever the blast heating equipment (stoves, etc.) are capable of delivering the required blast temperature.

The above-recited principles are further explained in the illustrative specific furnace data of the paragraphs immediately following:

A blast furnace with a 10 foot hearth, blown with 7650 cu. ft./min. of atmospheric air (measured at 60 F., 30 inches Hg, 60% humidity) at 4.5 lbs. pressure (gauge) preheated to 1800 F, was charged at minutes intervals with rounds comprising 3000 lbs. of coke (91.5% fixed carbon; 5.5% ash; 0.5% S) carrying a burden of 4210 lbs. of the above-described New Caledonia ore, 160 lbs. of sintered flue dust, 214 lbs. of slag (l. e., dust and slag from a prior operation of the furnace), 695 lbs. of limestone, 430 lbs. of quartzite rock (essentially, S102) and 32 gallons of water. The furnace discharged 28,000 cu. ft./min. of top gas at 920 F., actual volume (10,300 cu. ft./min.

dry basis, measured at 60 F. and 30" Hg) which analyzed: COP-2.23% by volume; C0-38.50%; Hz1.61%; Ns57.66%. The furnace cast 39,600 lbs. of metal at 6-hour intervals, and flushed 6,950 lbs. of slag at hourly intervals. The metal analysis was: (fr-63.64%; l c-27.35%; 81-

slag analysis was: Shh-27.94%; 24.00%; Mg0l8.28%; Geo-20.80%; Gnos- 10.34%; Foo-0.27%; 8-0.78%. Optical pyrometer readings (corrected for emissivity) on the metal cast (2840-2880 F.) and on the slag at flush (2880-2950 F.) indicated a mean hearth temperature of 2887 F.

When the blast temperature was raised to 2200" F. each item of the above burden was increased 31%, and metal production became 52,500 lbs. every 6 hours: hourly slag production. became 8600 lbs. Under these conditions the quality of the metal improved slightly, the metal analyzing: (Zr-64.90%; Fe 26.18%; C-7.45%; Si-l.25%; S0.04%; P-0.21%. The slag, then, carried away somewhat less chromium values, analyzing: Sim-28.78% Alcoa-25.24%; MEG-17.40%;

, Ca022.18%; Cr20:-6.58%; Foo-0.32%; S

0.68%. The 400 increase in blast temperature increased the tonnage of metal from 77 to net tons/day, decreased the coke consumption from 2800 to 2050, and-of greater practical importance-improved the handling of the furnace and made operation of the furnace less diflicult to maintain.

A similar improvement in operation and economy results from a further 400 increase in hot blast temperature to 2600 F.a temperature which I have been able to attain for short periods and which will become a metallurgical commonplace when improvements in blast heating equipment, now in development, are realized.

Continued operation of sorts" with the New Caledonia are c be attained in the furnace with blast temperatures as low as 1500 F. or even 1400 F.: but, the coke consumption becomes high, the daily tonnage is decreased, furnace irregularities become frequent and distressing, the

loss of chromium in the slag is excessive, the grade of metal is adversely aflected, and the fluidity of the slag is dimcult to control, with unfused unfluxed lumps of ore discharging into the hearth. Even under adverse conditions such as those concomitant with the lower ranges of blast temperature, the economy of the whole process is an improvement over the heretofore exclusive electric furnace production of ferro-chromium.

Illustration 2 A Russian chromite from the Urals shows an analysis Ci-203 55.8; FeO 21.6; MgO 13.9; A: 3.3; $102 5.4. After elimination of FeO by CO reduction in the shaft the oxide phase composition is: CrzOs 71.1; MgO 17.7; A120: 42; and S102 7.0. The MgO-CrzOa-AlzOa ternary composition is CrzOa 76.4; MgO 19.1; A1203 4.5; and the 7 projection on the MgO-AlaOa binary boundary at P0.24%. The

of 3800 F. The entropy of fusion of the ternary melt is 14.62'e. u. per mol and the eifect of the silica, present as 29 molar per cent of Forsterite (Mg SiOd depresses the solidus 195 F., producing a minimum solidus in the quaternary system MgO'CraO:-Ala0a-SiOa of 3547 R, which may be termed the temperature of initial liquefaction. It required (Figure 6) a blast temperature of 1710 F. to just reach this combustion zone temperature. For a desired 200 F. superheat a blast temperature of.2l F. is required.

The two illustrations of the determination of the liquefying temperature of chrome ores are sufficient to describe the principles used in determining the blast temperature required for the proper production of ferro-chromium in the blast furnace.

My invention, as defined herein, is the process of smelting ores containing a substantially high chromium content (i. e., of sufficient chromium content that the burden contains at least 15% of Or as Cl'zOa), in a blast furnace blown with natural atmospheric air (20.97% 02) with the specific restriction that the blast temperature is sufficiently high to raise the temperature of the solids in the combustion zone to an extent that substantial liquefaction of the ore particles is effected and a sufllcient superheat above the solidus is attained to permit fused ore to flow across the surfaces of the solid carbonaceous reducing agent particles or lumps, and hence to effect reduction of CrsOa of the ore to metallic chromium. In most of the pres available in practical work, the ratio of CrzOa to MgO, and of MgO to A1203, is suchthat the ore particle's oxide composition is very high melting, and I realize that temperatures higher than those heretofore employed in blast furnace practice are necessary in such cases. The necessary increase in combustion zone temperature might possibly be obtained by increasing the oxygen'content of the blast as-was described by Johnson. My present invention is confined to a blast furnace operation in which a blast of natural atmospheric air (freed from moisture, whenever the same can be done cheaply enough) is employed and in which the temperature of the blast preheat is sufllcient to efiect the desired elevated temperature in the combustion zone.

It should be noted that in supplying the blast furnace with the highly preheated blasts rec- I appended claims I'mean the total-charge of the blasting the furnace with atmospheric air preheated before entrance into the furnace to a temperature, not below 1800 F., controlled with respect to the ratio of carbonaceous fuel to said material, the temperature of preheat being increased as the ratio of carbonaceous fuel to said material is decreased. to maintain-in the furnace hearth a temperature sumciently elevated to effect liquefaction of the material and reduction of chromic oxide by solid carbon, whereby the greater part of the chromium content of the material is deoxidized and chromium alloy is problast furnace (minus the carbon contained in the duced; and removing the resulting liquid slag and liquid alloy from the furnace.

containing more than 35% chromium, which comprises charging into a blast furnace carbonaceous fuel and a material containing chromic oxide and a reducible oxygen compound of iron, said material containing at least of chromic oxide and less oxygen compound of iron than chromic oxide; blasting the furnace with atmospheric air preheated before entrance into the furnace to a temperature, not below 1800 F., controlled with respect to the ratio of carbonaceous fuel to said material, the temperature of preheat being increased as the ratio of carbonaceous fuel to said material is decreased, to maintain in the furnace hearth'a temperature suilicientiy elevated to effect liquefaction of the material and reduction of chromic oxide by solid carbon, whereby the greater part of the chromium content of the material is deoxidized and chromium alloy is produced; and removing the resulting slag and liquid alloy from. the furnace. 3. Process of metallizing chrome ore, which comprises charging the ore into a blast furnace with carbonaceous fuel and a flux; blasting the charge with air, containing less than oxygen, preheated to an elevated temperature, at least equal to 1800" F., controlled with respect to the ratio of ore to fuel employed in the charge the temperature of preheat being increased as the ratio of carbonaceous fuel to said material is decreased, whereby the hearth of the furnace is maintained at an elevated temperature greater than the blast temperature and sufliclently high with respect to the compositions of the metal and the slag produced therein to maintain metal and slag fluid; and removing from the hearth a slag containing less than 20% CH0: and a metallic product containing more than chromium.

4. Process of preparing ferro-chromium from a material containing chromic oxide, ferrous oxide, magnesia, alumina andsilica, wherein the ratio-of. CrzOa to FeO exceeds unity and wherein the sum of the metals chromium plus iron exceeds one-fourth of the total material, which comprises heating the material in a blast furnace in chemically reactive contact with a solid oarbonaceous reducing agent by means of a blast of atmospheric air preheated to a temperature of at least about 1800 F., whereby reduction of the greater part of the CH0: by carbon is effected to produce a fluid product containing more than 35% chromium in a deoxidized condition.

PERCY H. ROYS'I'ER. 

